One known method of detecting infrared radiation involves obtaining absorption of this radiation by a layer of small-gap semiconductor material such as CdHgTe for example. The absorption of photons by this layer then creates electron-hole pairs and the minority carriers thus produced diffuse or drift due to the effect of an electric field in the absorption layer so that they can subsequently be collected at the level of pn junctions then electrodes. Detection quality is then dictated primarily by the dimensions of the absorption layer.
In fact, the quantum efficiency of detection improves, the thicker this layer is. At the same time, the dark current, which is one of the main sources of noise with this type of detection, rises as the volume for collecting the photocarriers increases. Thus, designing such detection must strike a compromise between detection efficiency, as defined by the quantum efficiency, and the quantity of noise that is present in the signal obtained from detection which is partly generated by the dark current.
In order to overcome this problem, photodetectors of the Metal-Semiconductor-Metal (MSM) type having so-called metallic “plasmonic” structures on their absorption layer(s) have been proposed. Coupling between the incident electromagnetic radiation and surface plasmons is obtained in this way. Making an appropriate choice of said structures, which usually take the form of straight, parallel strips having a rectangular cross-section, then allows very considerable confinement of the electromagnetic field in the absorption layer, thus producing a significant reduction in the dark current.
Besides confining the electromagnetic field in a restricted space, plasmonic structures also make it possible, under certain geometric conditions, to reduce the response time of the detector which is related to the rate at which carriers diffuse into the absorption layer and they also make it possible to apply spectral filtering or polarization selectivity.
Document EP1247301 discloses a MSM photodetector with a plasmonic structure comprising metallic strips that alternate with semiconductor strips, this assembly being formed on an electrically insulating layer. The metallic strips form plasmonic structures and thus concentrate the electromagnetic field in the semiconductor strips. However, the metallic strips also act as an electrode for collecting photocarriers produced in the semiconductor strips.
The response time of such a photodetector is primarily dictated by the time that elapses between a photocarrier being created by the absorption of a photon by the semiconductor material and the instant at which that photocarrier is collected by the collecting electrodes. In order to reduce this collection time, it is consequently useful to reduce the time which it takes for the photocarriers to diffuse into the semiconductor material, in this case the semiconductor strips.
In the previous configuration, the geometry of the electrodes is imposed by the plasmonic resonance effect that is exploited and this geometry is determined and set by the wavelength that is to be detected. The way of reducing the collection time is then to use a so-called “interdigitated comb” architecture in which two consecutive strips are differently polarized, thus allowing collection by metal-semiconductor junctions formed where the strips come into contact with the absorbing material. The main drawback of this type of structure is the fact that it imposes two contacts per pixel in order to benefit from its maximum speed.
Similarly, Document FR 2 842 945 suggests forming metallic strips that form plasmonic structures on a semiconductor layer and providing a reflective layer underneath the semiconductor structure in order to obtain concentration of the electromagnetic field in the semiconductor layer that supports the metallic strips. The electric strips also act as collecting electrodes here, and so the same interfacing and wiring problem is encountered.